Virulence plasmid-associated sensitivity to acid in Escherichia coli and its possible significance in human infections.

Several strains of Escherichia coli were markedly sensitised to killing at pH 2.5 or 3.5 when the ColV,I-K94 virulence plasmid was introduced into them. For strain 1829, the effect on acid sensitivity was due to the presence of plasmid in the previously resistant strain rather than to its introduction into an acid-sensitive variant already in the population. Acid sensitivity was also conferred by the ColV-K30 and ColB-K98 plasmids and the resistance plasmid R124-F2; other plasmids tested had no marked effect. Studies of ColV+ strains carrying mutant plasmids indicated that it was the presence of ColV-encoded transfer components that made ColV,I-K94+ strains acid-sensitive. Organisms in the exponential phase of growth were more sensitive to acid than were those from stationary phase cultures and this difference was more marked for ColV,I-K94+ strains than for Col- ones. Moreover, ColV+ strains exposed to conditions of low pH for short periods subsequently grew less well than the Col- parent and appeared to be sensitised by the damage to the effects of H2O2. These results indicate that some ColV+ strains may be more sensitive to gastric acid and to phagocytic acidity than are Col- strains. ColV,I-K94+ strains grew as well as Col- ones in broth or urine at pH 4.5-6.0 which suggests that the presence of the plasmid would not be detrimental to bacterial growth in the urinary tract. The presence of transfer components in the outer membrane of ColV,I-K94+ bacteria may destabilize the lipopolysaccharide layer allowing increased penetration of hydrogen ions.